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Rubber Rx


May/June 2013 Volume Volume 6 Issue Issue 3


By Clare Goldsberry

(602) 996-6499

A medical device manufacturer discusses molding LSR


Image courtesy ProMed Molded Products

The micromolding of liquid silicone rubber is increasing because of LSR’s ability to accommodate difficult-to-mold geometries, such as undercuts, tapers and angles.

Moreover, manufacturers of medical parts and devices, in particular implantable device makers, have recognized a key benefit of LSR: it is biocompatible, so it can reside in the body for extended periods. The material is also flexible, solvent-resistant and exhibits thermal stability over a broad temperature range, making LSR medical parts suitable for sterilization.

However, the micromolding of LSR comes with challenges—just like the micromolding of thermoplastic-engineered resins. The challenges of molding LSR and thermoplastics are different, though (see sidebar, below). For example, thermoplastic parts can be readily removed from a mold via traditional, mechanical methods, such as ejector pins, whereas LSR parts—because of their soft, rubbery nature—require more delicate part removal and handling. Because the material is statically charged and tactile, a lot of thought and ingenuity must go into handling LSR components.

Molder’s perspective

Jason Nelson, new product development manager for ProMed Molded Products Inc., said that LSR’s flexibility makes it a good material for molding microparts. He noted, though, that “there’s no industry standard for defining ‘micro’ in terms of size. Because micro-silicone part dimensions are difficult to quantify, what we consider micro has more to do with weight—typically a gram weight of 0.0001 to 0.2. In that respect, probably 50 percent of what we do is micro.”

Since 1989, Minneapolis-based ProMed has been supporting medical OEMs in their efforts to miniaturize medical components and devices. A majority of the products ProMed develops are for implantable devices in applications such as cardiac-rhythm management, ophthalmology, neurology and drug delivery.


Roembke Mfg. & Design Inc. has built molds with total shot sizes of 0.015 g that provide the customers with flashless parts and no runner waste. Image courtesy Roembke Mfg. & Design.

Through the company’s ProMed Pharma LLC division, established in 2006, the company leverages its expertise in micromolded components by blending active pharmaceuticals into the silicone. One popular device is the “eye punctum plug,” used to treat dry eye, glaucoma and other eye diseases. Punctum plugs are smaller than a grain of rice. They have to be, Nelson said, because a physician inserts the drug-loaded plugs in the puncta (two small openings in the corners of the eyes, near the nose, through which tears drain). The plugs take the place of drops or pills used to treat eye ailments.

Silicone lends itself well to this application due to its soft, flexible nature. Pete Mangan, president of ProMed Molded Products, noted that there are two critical aspects of punctum plugs. They have to stay in the puncta for up to 90 days, and their small size makes them challenging to process.

“If a product like this is successful, you are looking at making and handling millions of tiny parts, and that is no easy task,” Mangan said.

Among the goals when molding microparts like these is to minimize human interaction during the manufacturing process and ensure cleanliness. ProMed manufactures these types of devices in an ISO 7 (Class 10,000) cleanroom environment.

“We usually operate at a much lower particle count than is allowed, which is key for our products because silicone is electrostatically charged and can quickly draw particles out of the air,” Mangan said.

Quality challenges

Silicone parts must be molded on the appropriate dedicated equipment.

“You have to use molding equipment designed specifically for LSR material and the size of the microcomponent,” said Mangan. “Having the right equipment is critical because the work material can often run from a couple hundred dollars a pound to several thousand dollars per pound. We use a variety of these machines to process silicone components for the medical and pharmaceutical industries. We’re looking to maximize parts per pound of material, and that requires an intricate dance between the machine, the mold and the operators.”

Nelson added it is difficult for ProMed to find molding equipment that does what the company needs, so it often customizes the machines it buys to ensure part accuracy and repeatability, and that they impart acceptable finishes.

ProMed has several captive moldmaking partners that build micromolds to its specifications. “You need extremely precise molding equipment to micromold the silicone,” Nelson said. Typically, it requires an electronic, servo-controlled injection unit to achieve the required precision. Then there’s the automation, which must provide ultraprecise accuracy and repeatability.



Specialty Silicone Fabricators makes molds for punctum plugs that are as small as 0.4mm. Image courtesy Specialty Silicone Fabricators.

Quality starts with the tooling, which must be built by a moldmaker with the right equipment and that has experience with the LSR molding process, according to Nelson. “Design features, such as gate location and vent sizes for tooling that molds parts this small, are extremely challenging,” he said. “Moldmakers need laser-venting capabilities, and these [vents] have to be accurate within tenths (10-thousandths of an inch). There are only a small number of toolmakers with the capabilities we require [to make the vents].”

Quality assurance and measurement is a huge challenge for LSR parts as well, and micro LSR multiplies those challenges.

“We have inline vision systems on some of our machines that perform in-process inspection, and we also do online audits throughout the molding process,”
Nelson said. “Because silicone is a soft material and any touch-type measurement system would distort the part, we use CMMs (coordinate measuring machines) with touch-free optics that have a high level of magnification to measure the parts.”

Because ProMed’s LSR parts can be colored or clear, the company uses two different types of optical measurement equipment, each of which can magnify parts up to 100×.

Nelson added that if a part has been designed for manufacturability and the right tooling and equipment is in place, a high-quality part will be produced.

“Proper tooling and proper equipment is the first step, and then it’s about the process,” said Nelson. “Our new-product development process is a scientific and defined process, with the goal being a consistent and repeatable process that results in high-quality parts.” µ

LSR and thermoplastics present different challenges

At first glance, molding liquid silicone rubber might seem similar to injection molding thermoplastics, but, in reality, the two are quite different.

The most significant difference is that thermoplastic materials are typically solid pellets that melt due to the high heat and friction of the screw in the barrel. Melting temperatures sometimes exceed 400° C, depending on the material being molded. The hot, molten material enters a mold. Cold water runs through cooling channels to quickly cool the molten plastic material—sometimes in a matter of seconds, depending upon the size and wall thickness of the part.


Shown here are several micromolded silicone punctum plugs. Some are as small as 0.060" long and 0.040" in diameter. Image courtesy M.R. Mold & Engineering.

LSR, on the on the other hand, is a two-component material. The separate materials—one of which contains a catalyst—are pumped from pails into a mixing box. These materials have a soft, gooey consistency and are mixed at a 1:1 ratio, with viscosities ranging from 50,000 cPs (1 centipoise = 0.01g/cm/second) to more than 1 million cPs, according to Nathan Wolfe, technical sales representative for medical implants at NuSil Silicone Technology, Carpinteria, Calif.

“In most cases, LSRs use a platinum-catalyzed addition-cure system and require a cure temperature above 80° C,” Wolfe explained. “Because of the viscosity of LSR, successful processing is dependent on using equipment to mix and pump thick materials without introducing air.”

Because of LSR’s thick, gooey consistency, it requires high pressures when injected into the mold. “Higher pressure is needed as the volume and number of cavities in a tool increases,” Wolfe said. Knowing how to control shrinkage during the vulcanization and curing stages is important, too. Generally, LSRs can be expected to shrink 2 to 3 percent, and the mold design should compensate for this, he added.

Rick Finnie, president of M.R. Mold & Engineering Inc., Brea, Calif., has spent 30 years building LSR molds. Commenting on the LSR injection-molding process, Finnie said that because silicone has a much lower viscosity than thermoplastics, it typically requires much lower injection pressures than thermoplastic materials.

“The problem comes at the end of the injection cycle, during the hold phase,” he explained. “With thermoplastics, you have material cooling and shrinking, so greater hold pressures are required to prevent sink marks. With silicone, the opposite is happening. The material is growing due to thermal expansion; therefore, very light hold pressure is used. This is also where you might cause the silicone to flash as well.”

He added that meeting a part’s dimensional requirements is a big challenge. In some cases, parts are intentionally molded with an overflow to ensure both dimensional accuracy and that the parts can be demolded.

“Shot size becomes super-critical in LSR components,” Finnie said. “Presently, few manufacturers make a molding machine that can process a microscale material shot. You really don’t worry about runner waste with microparts, because you may need to waste a little silicone material just to be able to get a dimensionally good part.”

Finnie said that due to the small size of the components, all molding steps have to be performed under magnification. “On these microscopic parts, quality is tough,” he added. “The slightest defect shows up like a glacier on the magnified part.”

—C. Goldsberry


M.R. Mold & Engineering Inc.
(714) 996-5511

NuSil Silicone Technology
(805) 684-8780

ProMed Molded Products Inc.
(763) 331-3800


Clare Goldsberry is a freelance writer who covers the injection molding and the moldmaking industry. Telephone: (602) 996-6499. E-mail: